Design Pamphlet for The Determination of Design Subgrade in Support of The 1993 Aashto Guide for The Design of Pavement Structures

FOREWORD

A key challenge faced by engineers using the 1993, AASHTO Guide for Design of Pavement
Structures (AASHTO Guide) is the selection of appropriate design values for the subgrade soil
and for the pavement materials. Until now, the information available to help engineers choose
appropriate values has been incomplete. This design pamphlet addresses this problem by
presenting procedures to characterize the subgrade soil. Two companion pamphlets — Design
Pamphlet for the Determination of Layered Elastic Moduli in Support of the 1993 AASHTO
Guide for the Design of Pavement Structures (FHWA-RD-97-077) and Design Pamphlet for the
Backcalculation of Pavement Layer Moduli in Support of the 1993 AASHTO Guide for Design
of Pavement Structures (FHWA-RD-97-076) — provide additional, related guidance on selecting
appropriate design values to characterize the pavement materials and interpretation of pavement
deflection data. The procedures presented were developed through the analysis of the Long-
Term Pavement Performance (LTPP) data, documented in the report Analyses Relating to
Pavement Material Characterization and Their Effects on Pavement Performance
(FHWA-RD-97-085).

Application of the procedures and guidelines developed through this analysis will facilitate and
improve application of the AASHTO Guide flexible pavement design procedures. Their use will
provide: (1) improved designs, (2) more realistic estimates of pavement performance, and (3)
more consistent use of the AASHTO design parameters. Furthermore, although the procedures
are specifically developed for use with the 1993 AASHTO Guide, their use will give agencies a
"leg up" on implementation of the design procedures being developed for inclusion in the 2002
AASHTO Guide for Design of New and Rehabilitated Pavement Structures. Thus, this pamphlet
and its companions are critically important to anyone who designs flexible pavements.

Charles J. Nemmers, P.E.
Director
Office of Engineering
Research and Development

Notice

This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its
contents or use thereof. This report does not constitute a standard, specification, or regulation.

The United States Government does not endorse products or manufacturers. Trademarks or
manufacturers' names appear herein only because they are considered essential to the object of
this document.